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Abstract:

An optical transmitter for an optical communication system includes a
light source that outputs optical signals having a plurality of
wavelengths, and a wavelength control unit. The wavelength control unit
receives an optical signal from the light source, resonates an optical
signal having a first wavelength, modulates the optical signal of the
first wavelength with a first transmission data signal to obtain an
intensity modulated optical signal, and outputs the intensity modulated
optical signal. The wavelength control unit may be integrally formed on a
semiconductor substrate in which a high thermal conductivity material is
used Alternatively, a trench that intercepts external heat may be formed
in a boundary surface of the wavelength control unit, and may be filled
with a low thermal conductivity material.

Claims:

1. An optical transmitter comprising: a light source that outputs optical
signals having a plurality of wavelengths; and a wavelength control unit
that receives an optical signal from the light source, resonates an
optical signal of the optical signals having a first wavelength,
modulates the optical signal of the first wavelength with a first
transmission data signal to obtain an intensity-modulated optical signal,
and outputs the intensity-modulated optical signal, wherein the
wavelength control unit is integrally formed on a semiconductor substrate
in which a high thermal conductivity material is used.

2. The optical transmitter of claim 1, further comprising a coupler
connected between the light source and the wavelength control unit that
stabilizes an optical signal output from the light source to the first
wavelength.

3. The optical transmitter of claim 2, wherein the wavelength control
unit comprises: a first waveguide through which the optical signals
received from the coupler is transmitted; a first prototype filter that
resonates the optical signal received from the first waveguide to the
first wavelength; a second waveguide that transmits the optical signal of
the first wavelength from the first prototype filter to the light source
through the coupler; third and fourth waveguides through which the
optical signal of the first wavelength received from the first prototype
filter is transmitted; and a first modulator that receives an optical
signal from the third waveguide and modulates an intensity of the optical
signal according to the first transmission data signal.

4. The optical transmitter of claim 3, wherein the wavelength control
unit monitors a power state of the light source using an optical signal
transmitted to the fourth waveguide.

5. The optical transmitter of claim 3, wherein the wavelength control
unit further comprises a second modulator that receives an optical signal
from the third waveguide and modulates a wavelength of the optical signal
according to a second transmission data signal.

6. The optical transmitter of claim 1, further comprising a circulator
connected between the light source and the wavelength control unit that
stabilizes an optical signal received from the light source to the first
wavelength, said circulator including a plurality of ports.

7. The optical transmitter of claim 6, wherein the wavelength control
unit comprises: a first waveguide that transmits an optical signal
received at a first port of the circulator from a second port of the
circulator; a first prototype filter that resonates an optical signal
received from the first waveguide to the first wavelength; a second
waveguide that transmits an optical signal of the first wavelength
received from the first prototype filter to a third port of the
circulator; third and fourth waveguides that transmit an optical signal
of the first wavelength received from the first prototype filter; and a
first modulator that receives an optical signal transmitted through the
third waveguide and modulates a wavelength of the optical signal
according to the first transmission data signal, wherein the circulator
transmits from the first port of the circulator to the light source the
optical signal received from the third port of the circulator.

8. The optical transmitter of claim 7, wherein the wavelength control
unit monitors a power state of the light source using an optical signal
transmitted to the fourth waveguide.

9. The optical transmitter of claim 7, wherein the wavelength control
unit comprises a second modulator that receives an optical signal from
the third waveguide and modulates a wavelength of the optical signal
according to a second transmission data signal.

11. The optical transmitter of claim 1, wherein the light source uses
amplified spontaneous emission (ASE), and the optical transmitter further
comprises a wavelength demultiplexer connected between the light source
and the wavelength control unit that divides an optical signal received
from the light source according to wavelengths.

12. The optical transmitter of claim 11, wherein the wavelength control
unit comprises: a first waveguide through which an optical signal
received from the wavelength demultiplexer is transmitted; a first
prototype filter that resonates an optical signal received from the first
waveguide to the first wavelength and transmits the optical signal to a
second waveguide; second and third waveguides through which an optical
signal of the first wavelength received from the first prototype filter
is transmitted; and a first modulator that receives an optical signal
from the second waveguide and modulates an intensity of the optical
signal according to the first transmission data signal.

13. The optical transmitter of claim 1, wherein the wavelength control
unit further comprises a trench that intercepts external heat formed in a
boundary surface thereof, said trench being filled with a low thermal
conductivity material.

14. An optical communication system comprising: a plurality of optical
transmitters that transmit optical data signals having different
wavelengths; a wavelength multiplexer that transmits to an optical
channel a wavelength-multiplexed optical signal formed from the optical
data signals received from each of the plurality of optical transmitters;
a wavelength demultiplexer that receives the wavelength-multiplexed
optical signal from the optical channel and divides the
wavelength-multiplexed optical signal according to wavelengths to obtain
wavelength-divided optical signals; and an optical receiver that converts
the wavelength-divided optical signals received from the wavelength
demultiplexer into electrical data signals, wherein each of the plurality
of optical transmitters comprises: a light source that outputs optical
signals having a plurality of wavelengths; and a wavelength control unit
that receives an optical signal from the light source, resonates an
optical signal of the optical signals having a first wavelength,
modulates the optical signal of the first wavelength with a first
transmission data signal to obtain an intensity-modulated optical signal,
and outputs the intensity-modulated optical wherein the wavelength
control unit is integrally formed on a semiconductor substrate in which a
high thermal conductivity material is used.

15. The optical communication system of claim 14, wherein the wavelength
control unit further comprises a trench that intercepts external heat
formed in a boundary surface thereof and filled with a low thermal
conductivity material.

16. An optical transmitter comprising: a light source that outputs
optical signals having a plurality of wavelengths; a wavelength control
unit that includes a prototype filter that resonates with a first
wavelength to transmit an optical signal having a first wavelength from
the optical signals, a first electrode disposed on an outer circumference
surface of the prototype filter, a second electrode disposed on an inner
circumference surface of the prototype filter, said first and second
electrodes adapted to receive a first transmission data signal that
modulates an intensity of the optical signal of the first wavelength,
wherein the wavelength control unit is integrally formed on a
semiconductor substrate; and a trench that intercepts external heat
formed in a boundary surface of wavelength control unit that is filled
with a low thermal conductivity material.

17. The optical transmitter of claim 16, further comprising a coupler
connected between the light source and the wavelength control unit that
stabilizes an optical signal output from the light source to the first
wavelength, wherein the wavelength control unit comprises: a first
waveguide through which the optical signals received from the coupler is
transmitted; a second prototype filter that resonates the optical signal
of the optical signals to the first wavelength; a second waveguide that
transmits the optical signal of the first wavelength from the second
prototype filter to the light source through the coupler; and third and
fourth waveguides through which the optical signal of the first
wavelength received from the first prototype filter is transmitted to the
first prototype filter.

18. The optical transmitter of claim 16, further comprising a circulator
connected between the light source and the wavelength control unit that
stabilizes an optical signal received from the light source to the first
wavelength, said circulator including a plurality of ports, wherein the
wavelength control unit comprises: a first waveguide that transmits an
optical signal received at a first port of the circulator from a second
port of the circulator; a second prototype filter that resonates an
optical signal received from the first waveguide to the first wavelength;
a second waveguide that transmits an optical signal of the first
wavelength received from the second prototype filter to a third port of
the circulator; and third and fourth waveguides that transmit an optical
signal of the first wavelength received from the second prototype filter
to the first prototype filter, wherein the circulator transmits from the
first port of the circulator to the light source the optical signal
received from the third port of the circular.

19. The optical transmitter of claim 16, wherein the light source uses
amplified spontaneous emission (ASE), and the optical transmitter further
comprises a wavelength demultiplexer connected between the light source
and the wavelength control unit that divides an optical signal received
from the light source according to wavelengths, wherein the wavelength
control unit further comprises: a first waveguide through which an
optical signal received from the wavelength demultiplexer is transmitted;
a second prototype filter that resonates an optical signal received from
the first waveguide to the first wavelength and transmits the optical
signal to a second waveguide; and second and third waveguides through
which an optical signal of the first wavelength received from the second
prototype filter is transmitted to the first prototype filter.

20. The optical transmitter of claim 16, wherein the wavelength control
unit further comprises: a second prototype filter that resonates an
optical signal received from the light sources to the first wavelength; a
second waveguide that transmits the optical signal of the first
wavelength received from the second prototype filter to the first
prototype filter; a third wave guide that receives the optical signal of
the first wavelength from the second prototype filter; and a modulator
that receives the optical signal from the third waveguide and modulates a
wavelength of the optical signal according to a second transmission data
signal.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of Korean Patent Application
No. 10-2012-0007796, filed on Jan. 26, 2012, in the Korean Intellectual
Property Office, the contents of which are herein incorporated by
reference in their entirety.

BACKGROUND

[0002] 1. Technical Field

[0003] Embodiments of the inventive concept are directed to an
optoelectronic integrated circuit, and more particularly, to an optical
transmitter and an optical communication system using a resonance
modulator that is thermally coupled.

[0004] 2. Discussion of the Related Art

[0005] Optical communication systems have been studied and developed to
increase the amount of data that may be transmitted. An optical
communication system uses an optical transmitter for transmitting
information through an optical fiber cable, and has been used primarily
for long-distance communication. However, as operating speeds of
electronic devices and the amount of transmitted data increase, optical
communication systems are being used for short-distance communication,
such as board-to-board or chip-to-chip communication.

SUMMARY

[0006] Embodiments of the inventive concept provide an optical transmitter
and an optical communication system using a resonance modulator that is
thermally coupled.

[0007] According to an aspect of the inventive concept, there is provided
an optical transmitter including: a light source that outputs optical
signals having a plurality of wavelengths; and a wavelength control unit
that receives an optical signal from the light source, resonates an
optical signal of the optical signals having a first wavelength,
modulates the optical signal of the first wavelength with a first
transmission data signal to obtain an intensity-modulated optical signal,
and outputs the intensity-modulated optical signal, wherein the
wavelength control unit is integrally formed on a semiconductor substrate
in which a high thermal conductivity material is used.

[0008] The optical transmitter may include a coupler connected between the
light source and the wavelength control unit that stabilizes an optical
signal output from the light source to the first wavelength.

[0009] The wavelength control unit may include: a first waveguide through
which the optical signal received from the coupler is transmitted; a
first prototype filter that resonates the optical signal received from
the first waveguide to the first wavelength; a second waveguide that
transmits the optical signal of the first wavelength received from the
first prototype filter to the light source through the coupler; third and
fourth waveguides through which the optical signal of the first
wavelength received from the first prototype filter is transmitted; and a
first modulator that receives an optical signal from the third waveguide
and modulates an intensity of the optical signal according to the first
transmission data signal.

[0010] The wavelength control unit may monitor a power state of the light
source by using an optical signal transmitted to the fourth waveguide.

[0011] The wavelength control unit may further include a second modulator
that receives an optical signal from the third waveguide and modulates a
wavelength of the optical signal according to a second transmission data
signal.

[0012] The optical transmitter may include a circulator connected between
the light source and the wavelength control unit that stabilizes an
optical signal received from the light source to the first wavelength.
The circulator may include a plurality of ports.

[0013] The wavelength control unit may include: a first waveguide that
transmits an optical signal received at a first port of the circulator
from a second port of the circulator; a first prototype filter that
resonates an optical signal received from the first waveguide to the
first wavelength; a second waveguide that transmits an optical signal of
the first wavelength received from the first prototype filter to a third
port of the circulator; third and fourth waveguides that transmit an
optical signal of the first wavelength received from the first prototype
filter; and a first modulator that receives an optical signal transmitted
through the third waveguide and modulates a wavelength of the optical
signal according to the first transmission data signal. The circulator
may transmit from the first port of the circulator to the light source
the optical signal received from the third port of the circulator.

[0014] The light source may be a distributed feedback laser diode (DFB-LD)
or a Fabry Perot laser diode (FP-LD).

[0015] The light source may use an amplified spontaneous emission (ASE),
and the optical transmitter may include a wavelength demultiplexer
connected between the light source and the wavelength control unit that
may divide an optical signal received from the light source according to
wavelengths.

[0016] The wavelength control unit may include: a first waveguide through
which an optical signal received from the wavelength demultiplexer is
transmitted; a first prototype filter that resonates an optical signal
received from the first waveguide to the first wavelength and transmits
the optical signal to a second waveguide; second and third waveguides
through which an optical signal of the first wavelength received from the
first prototype filter is transmitted; and a first modulator that
receives an optical signal from the second waveguide and modulates an
intensity of the optical signal according to the first transmission data
signal.

[0017] A trench that intercepts external heat transfer may be formed in a
boundary surface of the wavelength control unit, and a material having a
low thermal conductivity may be filled in the trench.

[0018] According to another aspect of the inventive concept, there is
provided an optical communication system including: a plurality of
optical transmitters that transmit optical data signals having different
wavelengths; a wavelength multiplexer that transmits to an optical
channel a wavelength-multiplexed optical signal formed from the optical
data signals received from each of the plurality of optical transmitters;
a wavelength demultiplexer that receives the wavelength-multiplexed
optical signal from the optical channel and divides the
wavelength-multiplexed optical signal according to wavelengths to obtain
wavelength-divided optical signals; and an optical receiver that converts
the wavelength-divided optical signals received from the wavelength
demultiplexer into electrical data signals, wherein each of the plurality
of optical transmitters includes: a light source that outputs optical
signals having a plurality of wavelengths; and a wavelength control unit
that receives an optical signal from the light source, resonates an
optical signal of the optical signals having a first wavelength,
modulates the optical signal of the first wavelength with a first
transmission data signal to obtain an intensity-modulated optical signal,
and outputs the intensity-modulated optical signal, wherein the
wavelength control unit is integrally formed on a semiconductor substrate
in which a high thermal conductivity material is used.

[0019] The wavelength control unit may include a trench that intercepts
external heat formed in a boundary surface thereof and filled with a low
thermal conductivity material.

[0020] According to another aspect of the inventive concept, there is
provided an optical transmitter including: a light source that outputs
optical signals having a plurality of wavelengths; a wavelength control
unit that includes a prototype filter that resonates with a first
wavelength to transmit an optical signal having a first wavelength from
the optical signals, a first electrode disposed on an outer circumference
surface of the prototype filter, a second electrode disposed on an inner
circumference surface of the prototype filter, said first and second
electrodes adapted to receive a first transmission data signal that
modulates an intensity of the optical signal of the first wavelength,
wherein the wavelength control unit is integrally formed on a
semiconductor substrate; and a trench that intercepts external heat
formed in a boundary surface of wavelength control unit that is filled
with a low thermal conductivity material.

[0021] The optical transmitter may include a coupler connected between the
light source and the wavelength control unit that stabilizes an optical
signal output from the light source to the first wavelength. The
wavelength control unit may include a first waveguide through which the
optical signals received from the coupler is transmitted; a second
prototype filter that resonates the optical signal of the optical signals
to the first wavelength; a second waveguide that transmits the optical
signal of the first wavelength from the second prototype filter to the
light source through the coupler; and third and fourth waveguides through
which the optical signal of the first wavelength received from the first
prototype filter is transmitted to the first prototype filter.

[0022] The optical transmitter may include a circulator connected between
the light source and the wavelength control unit that stabilizes an
optical signal received from the light source to the first wavelength,
said circulator including a plurality of ports. The wavelength control
unit may include a first waveguide that transmits an optical signal
received at a first port of the circulator from a second port of the
circulator; a second prototype filter that resonates an optical signal
received from the first waveguide to the first wavelength; a second
waveguide that transmits an optical signal of the first wavelength
received from the second prototype filter to a third port of the
circulator; and third and fourth waveguides that transmit an optical
signal of the first wavelength received from the second prototype filter
to the first prototype filter. The circulator may transmit from the first
port of the circulator to the light source the optical signal received
from the third port of the circular.

[0023] The light source may use amplified spontaneous emission (ASE), and
the optical transmitter further comprises a wavelength demultiplexer
connected between the light source and the wavelength control unit that
divides an optical signal received from the light source according to
wavelengths. The wavelength control unit may include a first waveguide
through which an optical signal received from the wavelength
demultiplexer is transmitted; a second prototype filter that resonates an
optical signal received from the first waveguide to the first wavelength
and transmits the optical signal to a second waveguide; and second and
third waveguides through which an optical signal of the first wavelength
received from the second prototype filter is transmitted to the first
prototype filter.

[0024] The wavelength control unit may include a second prototype filter
that resonates an optical signal received from the light sources to the
first wavelength; a second waveguide that transmits the optical signal of
the first wavelength received from the second prototype filter to the
first prototype filter; a third wave guide that receives the optical
signal of the first wavelength from the second prototype filter; and a
modulator that receives the optical signal from the third waveguide and
modulates a wavelength of the optical signal according to a second
transmission data signal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] FIG. 1 is a block diagram illustrating an optical communication
system including an optical transmitter, according to an embodiment of
the inventive concept.

[0026] FIG. 2 is a block diagram illustrating an optical transmitter
according to an embodiment of the inventive concept.

[0027]FIG. 3 is a block diagram illustrating an optical transmitter
according to another embodiment of the inventive concept.

[0028]FIG. 4 is a block diagram illustrating an optical transmitter
according to another embodiment of the inventive concept.

[0029]FIG. 5 is a block diagram illustrating an optical transmitter
according to another embodiment of the inventive concept.

[0030]FIG. 6 is a block diagram illustrating an optical transmitter
according to another embodiment of the inventive concept.

[0031] FIG. 7 is a block diagram illustrating an optical transmitter
according to another embodiment of the inventive concept.

[0032] FIG. 8 is a block diagram illustrating an optical transmitter
according to another embodiment of the inventive concept.

[0033]FIG. 9 is a block diagram illustrating an optical transmitter
according to another embodiment of the inventive concept.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0034] Embodiments of the inventive concept will now be described more
fully with reference to the accompanying drawings, in which exemplary
embodiments of the inventive concept are shown. However, this is not
intended to limit embodiments of the inventive concept to particular
modes of practice, and it is to be appreciated that all changes,
equivalents, and substitutes that do not depart from the spirit and
technical scope of the inventive concept are encompassed in the inventive
concept. Like reference numerals denote like elements in the drawings.

[0035] A large-capacity optical communication network may use
wavelength-division multiplexing (WDM) in which a plurality of
wavelengths are multiplexed and transmitted from a transmitter and then
split apart at a receiver.

[0036] FIG. 1 is a block diagram illustrating an optical communication
system 100 including an optical transmitter 111, according to an
embodiment of the inventive concept.

[0037] Referring to FIG. 1, the optical communication system 100 includes
the optical transmitter 111, a wavelength multiplexer 112, an optical
channel 121, a wavelength demultiplexer 131, and an optical receiver 132.

[0038] The optical transmitter 111 may use as a light source a distributed
feedback laser diode (DFB-LD) or a Fabry Perot laser diode (FP-LD), which
is a multi-wavelength light source. Alternatively, the optical
transmitter 111 may use amplified spontaneous emission (ASE) as a light
source. The optical transmitter 111 may include a plurality of channels.
Each of the channels may receive an optical signal having a desired
wavelength λ and modulate the optical signal according to a
transmission data signal.

[0039] The wavelength multiplexer 112 may pass therethrough optical
signals having different wavelengths λ1, . . . , and
λn transmitted from the optical transmitter 111. The
wavelength multiplexer 112 may use an arrayed waveguide grating (AWG).
The wavelength multiplexer 112 may distribute the optical signals to the
arrayed waveguides of the AWG. The AWG may be a waveguide circuit
fabricated by depositing quartz-based glass or silicon on a substrate
formed of silicon or the like. The optical signals propagating through
the wavelength multiplexer 112 may be transmitted through the optical
channel 121.

[0040] The optical channel 121 may transmit the optical signals by using
an integrated planar waveguide, an optical waveguide, or an optical
fiber. In wavelength-division multiplexing (WDM), optical signals may
effectively use the wide band capacity provided by an optical fiber. WDM
transmitted signals may have greater bandwidth than time-division
multiplexing (TDM) transmitted signals because WDM transmits signals
according to the number of divided wavelengths.

[0041] The optical channel 121 may reduce interaction between channels by
reducing a walk-off length by using an optical fiber having a large
dispersion. The optical channel 121 may reduce a nonlinearity coefficient
by using an optical fiber having a large effective core area. Also, the
optical channel 121 may reduce nonlinear effects of the light intensity
by setting the intensity of light transmitted to the optical fiber to a
lowermost value in an allowable range.

[0042] The wavelength demultiplexer 131 may receive an optical signal
formed using WDM and transmitted through the optical channel 121 and
divide the optical signal according to its wavelengths. The wavelength
demultiplexer 131 may use an AWG. The optical signal propagating through
the wavelength demultiplexer 131 may be transmitted to the optical
receiver 132. The optical receiver 132 may convert the optical signal
into an electrical signal that is original transmitted data.

[0043] FIG. 2 is a block diagram illustrating an optical transmitter 111A
according to an embodiment of the inventive concept.

[0044] Referring to FIG. 2, the optical transmitter 111A may transmit
optical signals output from a plurality of channels CH1, . . . , and CHn
to the wavelength multiplexer 112 through waveguides 213 and 214. The
optical transmitter 111A may respectively transmit optical signals having
different wavelengths λ1, . . . , and λn through
the channels CH1, . . . , and CHn. The optical transmitter 111A may
include a plurality of light sources 201 and 202 and wavelength control
units 203 and 204. The wavelength control units 203 and 204 may be called
modulators 203 and 204. The optical transmitter 111A may be connected to
the optical channel 121 through the wavelength multiplexer 112 using an
AWG.

[0045] The first channel CH1 may include the light source 201 and the
modulator 203. A DFB-LD may be used as the light source 201. A DFB-LD is
a multi-wavelength light source having a very narrow frequency line
width, but is relatively expensive. Alternatively, an FP-LD may be used
as the light source 201.

[0046] The modulator 203 may include a prototype filter 208, a first
electrode 209, a second electrode 210, and a heater 211. The prototype
filter 208 may resonate for an optical signal having a specific
wavelength. The prototype filter 208 may have a resonance wavelength of,
for example, a first wavelength λ1. The prototype filter 208
may output an optical signal having the first wavelength λ1
from the light source 201.

[0047] The first electrode 209 may be disposed on an outer circumferential
surface of the prototype filter 208, and the second electrode 210 may be
disposed on an inner circumferential surface of the prototype filter 208.
A first transmission data signal ES1 is a binary signal that may be
applied to the first electrode 209 and the second electrode 210. For
example, if the first transmission data signal ES1 is a logic low signal
having a ground voltage level, there is no voltage difference between the
first electrode 209 and the second electrode 210. If the first
transmission data signal ES1 is a logic high signal having a
predetermined voltage level, there is a predetermined voltage difference
between the first electrode 209 and the second electrode 210.

[0048] An intensity of an optical signal of first wavelength output from
the prototype filter 208 may be modulated by to a voltage difference
between the first electrode 209 and the second electrode 210 due to the
first transmission data signal ES1. When a logic low first transmission
data signal ES1 is applied, with no voltage difference between the first
and second electrodes 209 and 210, the prototype filter 208 resonates at
the first wavelength λ1 and maximizes an intensity of an
optical signal output from the prototype filter 208. When a logic high
first transmission data signal ES1 is applied, with a predetermined
voltage difference between the first and second electrodes 209 and 210,
the prototype filter 208 resonates at a wavelength shifted from the first
wavelength λ1 minimizing an intensity of an optical signal
output from the prototype filter 208.

[0049] An optical signal modulated according to the first transmission
data signal ES1 and output from the prototype filter 208 may be
transmitted to the wavelength multiplexer 112 through the waveguide 213.

[0050] The first wavelength λ1, which is a resonance wavelength
of the prototype filter 208, can vary due to a temperature change. To
maintain the first wavelength λ1 of the prototype filter 208
irrespective of temperature, the heater 211 may be disposed over the
prototype filter 208. A temperature of the prototype filter 208 may be
kept constant by maintaining a constant temperature from the heater 211.

[0051] Like the first channel CH1, other channels of the optical
transmitter 111A each may include a light source and a modulator. For
example, the nth channel CHn may include the light source 202 having
a DFB-LD, and the modulator 204 that modulates an optical signal output
from the light source 202. A prototype filter in the modulator 204 may
have a resonance wavelength of, for example, an nth wavelength
λn. The modulator 204 may receive an optical signal of
nth wavelength λn output from the light source 202 and
modulate the optical signal of nth wavelength λn
according to an nth transmission data signal ESn. An optical signal
modulated according to the nth transmission data signal ESn and
output from the modulator 204 may be transmitted to the wavelength
multiplexer 112 through the waveguide 214. To maintain the nth
wavelength λn, which is a resonance wavelength of the
prototype filter 208 in the modulator 204, irrespective of temperature,
the modulator 204 may include a heater.

[0052]FIG. 3 is a block diagram illustrating an optical transmitter 111B
according to another embodiment of the inventive concept.

[0053] Referring to FIG. 3, the optical transmitter 111B may transmit
optical signals from the plurality of channels CH1, . . . , and CHn to
the wavelength multiplexer 112 through waveguides 313 and 323. The
optical transmitter 111B may respectively transmit optical signals having
the different wavelengths λ1, . . . , and λn to the
channels CH1, . . . , and CHn. The optical transmitter 111B may include a
plurality of light sources 301 and 321, a plurality of couplers 302 and
322, and a plurality of wavelength control units 314 and 324. The optical
transmitter 111B may be connected to the optical channel 121 through the
wavelength multiplexer 112 using an AWG.

[0054] The first channel CH1 may include the light source 301, the coupler
302, and the wavelength control unit 314. An FP-LD may be used as the
light source 301. An FP-LD is relatively inexpensive, and has a narrow
frequency line width, as shown in FIG. 4. Alternatively, a DFB-LD may be
used as the light source 301.

[0055] The coupler 302 may receive and distribute an optical signal output
from the light source 301. Also, the coupler 302 may receive and output
an optical component of an optical signal that is backscattered or
reflected. The coupler 302 may be a bidirectional coupler. An optical
signal passing through the coupler 302 may be transmitted to the
wavelength control unit 314.

[0056] The wavelength control unit 314 may modulate an intensity of an
optical signal transmitted through the coupler 302 according to the first
transmission data signal ES1. The wavelength control unit 314 may include
a first waveguide 303, a first prototype filter 304, a second waveguide
305, a third waveguide 306, a fourth waveguide 307, and a modulator 311.

[0057] An optical signal input to the wavelength control unit 314 through
the coupler 302 may be transmitted to the first and second waveguides 303
and 305. An optical signal transmitted to the first waveguide 303 may be
transmitted to the first prototype filter 304. The first prototype filter
304 may have a resonance wavelength of, for example, the first wavelength
An optical component having the first wavelength λ1 that
matches a resonance curve of the first prototype filter 304 may be output
from the first waveguide 303 to the second waveguide 305. An optical
signal of first wavelength λ1 output to the second waveguide
305 may be transmitted back to the light source 301 through the coupler
302. Through this process, an optical signal output from the light source
301 may be stabilized to the first wavelength λ1.

[0058] An optical signal transmitted to the second waveguide 305 through
the coupler 302 may be transmitted to the first prototype filter 304. An
optical component having the first wavelength λ1 that matches
the resonance curve of the first prototype filter 304 may be output from
the second waveguide 305 to the first waveguide 303. An optical signal of
first wavelength λ1 output to the first waveguide 303 may be
transmitted back to the light source 301 through the coupler 302. Through
this process, an optical signal output from the light source 301 may be
further stabilized to the first wavelength λ1.

[0059] An optical signal of first wavelength λ1 output from the
first prototype filter 304 may be transmitted to the third waveguide 306
and the fourth waveguide 307. An optical signal transmitted to the third
waveguide 306 may be provided to the modulator 311 to be modulated. The
modulator 311 may modulate an intensity of the optical signal according
to the first transmission data signal ES1.

[0060] An optical signal transmitted to the fourth waveguide 307 may be
used to monitor a power state of the light source 301. In detail, when
the power of the optical signal transmitted to the fourth waveguide 307
is lower than an upper limit, the power of the light source 301 may be
increased, and when the power of the optical signal transmitted to the
fourth waveguide 307 is higher than the upper limit, the power of the
light source 301 may be reduced. The power of the light source 301 may be
monitored without interrupting its transmission.

[0061] The modulator 311 may include a second prototype filter 308, a
first electrode 309, and a second electrode 310. The second prototype
filter 308 may have a resonance wavelength of the first wavelength, like
the first prototype filter. 304. The first electrode 309 may be disposed
on an outer circumferential surface of the second prototype filter 308,
and the second electrode 310 may be disposed on an inner circumferential
surface of the second prototype filter 308.

[0062] The binary first transmission data signal ES1 may be applied to the
first electrode 309 and the second electrode 310. For example, when the
first transmission data signal ES1 is a logic low signal having a ground
voltage level, there is no voltage difference between the first electrode
309 and the second electrode 310. When the first transmission data signal
ES1 is a logic high signal having a predetermined voltage level, there is
a predetermined voltage difference between the first electrode 309 and
the second electrode 310.

[0063] The second prototype filter 308 receives the optical signal
transmitted to the third waveguide 306 and outputs an optical component
having the first wavelength λ1 that matches a resonance curve
of the second prototype filter 308. An intensity of an optical signal
output from the second prototype filter 308 may be modulated by a voltage
difference between the first and second electrodes 309 and 310 due to the
first transmission data signal ES1.

[0064] When a logic low first transmission data signal ES1 is applied to
the first and second electrodes 309 and 310, an intensity of an optical
signal output from the second prototype filter 308 is maximized. When a
logic high first transmission data signal ES1 is applied to the first and
second electrodes 309 and 310, an intensity of an optical signal output
from the second prototype filter 308 is minimized.

[0065] An optical signal modulated according to the first transmission
data signal ES1 and output from the second prototype filter 308 may be
transmitted to the wavelength multiplexer 112 through the fifth waveguide
313.

[0066] The first and second waveguides 303 and 305, the first prototype
filter 304, the third and fourth waveguides 306 and 307, and the
modulator 311 may be thermally coupled to one another to have the same
temperature. A refractive index of each of the first and second
waveguides 303 and 305 and the third and fourth waveguides 306 and 307
may vary due to an ambient temperature change. The first wavelength
λ1 may vary due to an ambient temperature change.

[0067] For an optical signal that is stably resonated to the first
wavelength λ1 to be input to the modulator 311, the thermally
coupled first and second waveguides 303 and 305, the first prototype
filter 304, the third and fourth waveguides 306 and 307, and the
modulator 311 may be integrally formed as one wavelength control unit 314
on a semiconductor substrate. A material having a high thermal
conductivity, such as silicon nitride, may be used in the wavelength
control unit 314, or a trench that intercepts external heat may be formed
in a boundary surface of the wavelength control unit 314 and be filled
with a material having a low thermal conductivity.

[0068] Like the first channel CH1, other channels of the optical
transmitter 111B may each include a light source, a coupler, and a
wavelength control unit. For example, the nth channel CHn may
include the light source 321 for which an FP-LD may be used, the coupler
322 that receives and distributes an optical signal output from the light
source 321, and the wavelength control unit 324 that modulates an optical
signal transmitted through the coupler 322 according to the nth
transmission data signal ESn.

[0069] Prototype filters in the wavelength control unit 324 may have a
resonance wavelength of, for example, the nth wavelength
λn. The wavelength control unit 324 may receive an optical
signal of nth wavelength λn, output from the light source
321 and modulate the optical signal according to the nth
transmission data signal ESn. An optical signal modulated according to
the nth transmission data signal ESn and output from the wavelength
control unit 324 may be transmitted to the wavelength multiplexer 112
through the sixth waveguide 323.

[0070] The wavelength control unit 324 may be integrated onto a
semiconductor substrate to provide the same temperature for waveguides
and the prototype filters in the wavelength control unit 324. A material
having a high thermal conductivity may be used in the wavelength control
unit 324, and a trench that intercepts external heat may be formed in a
boundary surface of the wavelength control unit 324 and filled with a
material having a low thermal conductivity.

[0071]FIG. 5 is a block diagram illustrating an optical transmitter 111C
according to another embodiment of the inventive concept.

[0072] Referring to FIG. 5, the optical transmitter 111C is similar to the
optical transmitter 111B of FIG. 3, except that optical signals of the
channels CH1, . . . , and CHn transmitted to the optical channel 121
through the fifth and sixth waveguides 313 and 323 and the wavelength
multiplexer 112 in the optical transmitter 111B of FIG. 3 are directly
transmitted to the optical channel 121 in the optical transmitter 111C of
FIG. 5. A detailed description of the elements of the optical transmitter
111C that are the same as corresponding elements in FIG. 3 will not be
repeated.

[0073] In the optical transmitter 111C, an optical signal of first
wavelength λ1 modulated according to the first transmission
data signal ES1 of the first channel CH1 and output from the wavelength
control unit 314 may be directly transmitted to the optical channel 121.
An optical signal of nth wavelength λn modulated
according to the nth transmission data signal ESn of the nth channel
CHn and output from the wavelength control unit 324 may be directly
transmitted to the optical channel 121.

[0074] The optical channel 121 may act as one optical waveguide that
transmits optical signals having the first through nth wavelengths
λ1, . . . , and λn. The optical channel may
equalize intensities of the optical signals having the first through
nth wavelengths λ1, . . . , and λn.

[0075]FIG. 6 is a block diagram illustrating an optical transmitter 111D
according to another embodiment of the inventive concept.

[0076] Referring to FIG. 6, the optical transmitter 111D is similar to the
optical transmitter 111B of FIG. 3, except that to stabilize wavelengths
output from the FP-LD light sources 301 and 321, the optical transmitter
111D uses circulators 602 and 622 instead of the couplers 302 and 322 of
the optical transmitter 111B of FIG. 3. A detailed explanation of the
elements of the optical transmitter 111D that are the same as
corresponding elements in FIG. 3 will not be repeated.

[0077] In the first channel CH1, an optical signal output from the light
source 301 may be transmitted to the circulator 602. The circulator 602
is a passive nonreciprocal device including three or more ports. For
example, if the circulator 602 includes three ports, the circulator 602
may be configured such that light input to a first port is output from a
second port, light input to the second port is output from a third port,
and light input to the third port is output from the first port.

[0078] The circulator 602 may operate based on a nonreciprocal phase shift
or a Faraday rotation. The circulator 602 may include center electrodes
which intersect at a predetermined angle on a ferrite sheet A static
magnetic field may be applied to the ferrite sheet, and a high frequency
magnetic field may be generated by the center electrodes using
ferromagnetic characteristics of the ferrite sheet. Nonreciprocal
characteristics are obtained by rotating a polarization plane of the high
frequency magnetic field.

[0079] An optical signal output from the light source 301 may be input to
the first port of the circulator 602, output from the second port of the
circulator 602, and transmitted to the first waveguide 303. An optical
signal transmitted to the first waveguide 303 may be transmitted to the
first prototype filter 304, and an optical component having the first
wavelength λ1 that matches the resonance curve of the first
prototype filter 304 may be transmitted to the second waveguide 305. An
optical signal transmitted to the second waveguide 305 may be input to
the third port of the circulator 602, output from the first port of the
circulator 602, and transmitted back to the light source 301. Through
this process, an optical signal output from the light source 301 may be
stabilized to the first wavelength λ1.

[0080] FIG. 7 is a block diagram illustrating an optical transmitter 111E
according to another embodiment of the inventive concept.

[0081] Referring to FIG. 7, the optical transmitter 111E has at least two
transmission data signal groups for modulating optical signals having the
wavelengths λ1, . . . , and λn, thereby expanding
the number of channels. The optical transmitter 111E may transmit optical
signals output from the plurality of channels CH1, . . . , and CHn to a
first wavelength multiplexer 112A and a second wavelength multiplexer
112B. The optical transmitter 111E may transmit optical signals modulated
by a first transmission data signal group ES1A, . . . , and ESnA to the
first wavelength multiplexer 112A and transmit optical signals modulated
by a second transmission data signal group ES1B, . . . , and ESnB to the
second wavelength multiplexer 112B.

[0082] The optical transmitter 111E may include a plurality of light
sources 701 and 721, a plurality of couplers 702 and 722, and a plurality
of wavelength control units 714 and 724. The optical transmitter 111E may
be connected to first and second optical channels 121A and 121B through
the first and second wavelength multiplexers 112A and 112B each using an
AWG.

[0083] The first channel CH1 may include the light source 701, the coupler
702, and the wavelength control unit 714. An FP-LD or a DFB-LD may be
used as the light source 701. The coupler 702 may receive and distribute
an optical signal output from the light source 701. Also, the coupler 702
may receive and output an optical component of an optical signal which is
backscattered or reflected. The coupler 702 may be a bidirectional
coupler. An optical signal propagating through the coupler 702 may be
transmitted to the wavelength control unit 714.

[0084] The wavelength control unit 714 may modulate an optical signal
transmitted through the coupler 702 according to a first transmission
data signal ES1A of the first transmission data signal group ES1A, . . .
, and ESnA. The wavelength control unit 714 may modulate an optical
signal transmitted through the coupler 702 according to a first
transmission data signal ES1B of the second transmission data signal
group ES1B, . . . , and ESnB. The wavelength control unit 714 may include
a first waveguide 703, a first prototype filter 704, a second waveguide
705, a third waveguide 706, a fourth waveguide 707, a first modulator
711, and a second modulator 718.

[0085] An optical signal input to the wavelength control unit 714 through
the coupler 702 may be transmitted to the first and second waveguides 703
and 705. An optical signal transmitted to the first waveguide 703 may be
transmitted to the first prototype filter 704. The first prototype filter
704 may have a resonance wavelength of, for example, the first wavelength
λ1. An optical component having the first wavelength
λ1 that matches a resonance curve of the first prototype
filter 704 may be output from the first waveguide 703 to the second
waveguide 705. An optical signal of first wavelength λ1 output
to the second waveguide 705 may be transmitted back to the light source
702 through the coupler 702. Through this process, an optical signal
output from the light source 701 may be stabilized to the first
wavelength λ1.

[0086] An optical signal transmitted to the second waveguide 705 through
the coupler 702 may be transmitted to the first prototype filter 704. An
optical component having the first wavelength λ1 that matches
the resonance curve of the first prototype filter 704 may be output from
the second waveguide 705 to the first waveguide 703. An optical signal of
first wavelength λ1 output to the first waveguide 703 may be
transmitted back to the light source 701 through the coupler 702. Through
this process, an optical signal output from the light source 701 may be
further stabilized to the first wavelength λ1.

[0087] An optical signal of first wavelength λ1 output from the
first prototype filter 704 may be transmitted to the third waveguide 706
and the fourth waveguide 707. An optical signal transmitted to the third
waveguide 706 may be provided to the first modulator 711 to be optically
modulated. The first modulator 711 may modulate an intensity of the
optical signal according to the first transmission data signal ES1A of
the first transmission data signal group ES1A, . . . , and ESnA. An
optical signal transmitted to the fourth waveguide 707 may be provided to
the second modulator 718 to be optically modulated. The second modulator
718 may modulate an intensity of the optical signal according to the
first transmission data signal ES1B of the second transmission data
signal group ES1B, . . . , and ESnB.

[0088] The first modulator 711 may include a second prototype filter 708,
a first electrode 709, and a second electrode 710. The second prototype
filter 708 may have a resonance wavelength of the first wavelength
λ1, like the first prototype filter 704. The first electrode
709 may be disposed on an outer circumferential surface of the second
prototype filter 708, and the second electrode 710 may be disposed on an
inner circumferential surface of the second prototype filter 708. The
first transmission data signal ES1A is a binary signal that may be
applied to the first electrode 709 and the second electrode 710.

[0089] The second prototype filter 708 may receive an optical signal
transmitted to the third waveguide 706 and outputs an optical component
having the first wavelength λ1 that matches a resonance curve
of the second prototype filter 708. An intensity of an optical signal of
first wavelength λ1 output from the second prototype filter
708 may be modulated by a voltage difference between the first and second
electrodes 709 and 710 due to the first transmission data signal ES1A.

[0090] When a logic low first transmission data signal ES1A is applied to
the first and second electrodes 709 and 710, an intensity of an optical
signal output from the second prototype filter 708 is maximized. When a
logic high first transmission data signal ES1A is applied to the first
and second electrodes 709 and 710, an intensity of an optical signal
output from the second prototype filter 708 is minimized.

[0091] The second modulator 718 may include a third prototype filter 715,
a third electrode 716, and a fourth electrode 717. The third prototype
filter 715 may have a resonance wavelength of the first wavelength
λ1, like the first prototype filter 704. The third electrode
716 may be disposed on an outer circumferential surface of the third
prototype filter 715, and the fourth electrode 717 may be disposed on an
inner circumferential surface of the third prototype filter 715. The
first transmission data signal ES1B is a binary signal that may be
applied to the third electrode 716 and the fourth electrode 717.

[0092] The third prototype filter 715 may receive an optical signal
transmitted to the fourth waveguide 707 and output an optical component
having the first wavelength λ1 that matches a resonance curve
of the third prototype filter 715. A wavelength of an optical signal of
first wavelength λ1 output from the third prototype filter 715
may be modulated by a voltage difference between the third and fourth
electrodes 716 and 717 due to the first transmission data signal ES1B.

[0093] When a logic low first transmission data signal ES1B is applied to
the third and fourth electrodes 716 and 717, an intensity of an optical
signal output from the third prototype filter 715 is maximized. When a
logic high first transmission data signal ES1B is applied to the third
and fourth electrodes 716 and 717, an intensity of an optical signal
output from the third prototype filter 715 is minimized.

[0094] An optical signal modulated according to the first transmission
data signal ES1B of the second transmission data signal group ES1B, . . .
, and ESnB and output from the third prototype filter 715 may be
transmitted to the second wavelength multiplexer 112B through a sixth
waveguide 719.

[0095] The first and second waveguides 703 and 705, the first prototype
filter 704, the third and fourth waveguides 706 and 707, and the first
and second modulators 711 and 718 may be thermally coupled to one another
to maintain the same temperature. The refractive index of each of the
first, second, third, and fourth waveguides 703, 705, 706 and 707 may
vary due to an ambient temperature change. The first wavelength the
resonance wavelength of each of the first, second and third prototype
filters 704, 711 and 718, may vary due to an ambient temperature change.

[0096] For stably resonated optical signals of first wavelength to be
input to the first and second modulators 711 and 718, the thermally
coupled first and second waveguides 703 and 705, first prototype filter
704, third and fourth waveguides 706 and 707, and first and second
modulators 711 and 718 may be integrally formed as one wavelength control
unit 714 on a semiconductor substrate. A material having a high thermal
conductivity may be used in the wavelength control unit 714, or a trench
that intercepts external heat may be formed in a boundary surface of the
wavelength control unit 714 and filled with a material having a low
thermal conductivity.

[0097] Like the first channel CH1, other channels of the optical
transmitter 111E may each include a light source, a coupler, and a
wavelength control unit. For example, the nth channel CHn may
include an FP-LD light source 721, the coupler 722 that receives and
distributes an optical signal output from the light source 721, and the
wavelength control unit 724 that modulates an optical signal transmitted
through the coupler 722 according to the nth transmission data
signal ESn.

[0098] Prototype filters in the wavelength control unit 724 may have a
resonance wavelength of, for example, the nth wavelength
λn. The wavelength control unit 724 may include first and
second modulators that receive an optical signal of nth wavelength
λn output from the light source 321. The first modulator may
modulate an intensity of an optical signal according to an nth
transmission data signal ESnA of the first transmission data signal group
ES1A, . . . , and ESnA. The second modulator may modulate an intensity of
an optical signal according to an nth transmission data signal ESnB
of the second transmission data signal group ES1B, . . . , and ESnB.

[0099] An optical signal modulated according to the nth transmission
data signal ESnA of the first transmission data signal group ES1A, . . .
, and ESnA and output from the first modulator of wavelength control unit
724 may be transmitted to the first wavelength multiplexer 112A. An
optical signal modulated according to the nth transmission data
signal ESnB of the second transmission data signal group ES1B, . . . ,
and ESnB and output from the second modulator wavelength control unit 724
may be transmitted to the second wavelength multiplexer 112B.

[0100] The wavelength control unit 724 may be integrated onto a
semiconductor substrate to maintain the same temperature for the
waveguides and prototype filters in the wavelength control unit 724. A
high thermal conductivity material may be used in the wavelength control
unit 724, or a trench that intercepts external heat may be formed in a
boundary surface of the wavelength control unit 724 and filled with a low
thermal conductivity material.

[0101] In the first through nth channels CH1, . . . , and CHn,
optical signals propagating through the first wavelength multiplexer 112A
may be transmitted to the first optical channel 121A, and optical signals
propagating through the second wavelength multiplexer 112B may be
transmitted to the second optical channel 121B. The first transmission
data signal group ES1A, . . . , and ESnA may be a band including the
different wavelengths λ1, . . . , and λn, and the
second transmission data signal group ES1B, . . . , and ESnB may be a
band including the different wavelengths λ1, . . . , and
λn. Accordingly, since the optical transmitter 111E includes
the expanded optical channels 121A and 121B, the amount of data that may
be transmitted may be increased.

[0102] FIG. 8 is a block diagram illustrating an optical transmitter 111F
according to another embodiment of the inventive concept.

[0103] Referring to FIG. 8, the optical transmitter 111F is similar to the
optical transmitter 111E of FIG. 7, except that to stabilize wavelengths
output from the light sources 701 and 702, the optical transmitter 111F
uses circulators 802 and 822, instead of the couplers 702 and 722 of the
optical transmitter 111E of FIG. 7. A detailed explanation of the
elements of the optical transmitter 111F that are the same as
corresponding elements in FIG. 7 will not be repeated.

[0104] An optical signal output from the light source 701 may be
transmitted to the circulator 802. The circulator 802 is a passive
nonreciprocal device including three or more ports. For example, if the
circulator 802 includes three ports, the circulator 802 is configured
such that light input to a first port is output from a second port, light
input to the second port is output from a third port, and light input to
the third port is output from the first port.

[0105] The circulator 802 may operate based on nonreciprocal phase shift
or Faraday rotation. The circulator 802 may include center electrodes
which intersect each other at a predetermined angle may on a ferrite
sheet. A static magnetic field may be applied to the ferrite sheet, and a
high frequency magnetic field may be generated by the center electrodes
using ferromagnetic characteristics of the ferrite sheet Nonreciprocal
characteristics are obtained by rotating a polarization plane of the high
frequency magnetic field.

[0106] An optical signal output from the light source 701 may be input to
the first port of the circulator 802, output from the second port of the
circulator 802, and transmitted to the first waveguide 703. An optical
signal transmitted to the first waveguide 703 may be transmitted to the
first prototype filter 704, and an optical component having the first
wavelength λ1 that matches a resonance curve of the first
prototype filter 804 may be output to the second waveguide 705. An
optical signal transmitted to the second waveguide 805 may be input to
the third port of the circulator 802, output from the first port of the
circulator 802, and transmitted back to the light source 701. Through
this process, an optical signal output from the light source 701 may be
stabilized to the first wavelength λ1.

[0107]FIG. 9 is a block diagram illustrating an optical transmitter 111G
according to another embodiment of the inventive concept.

[0108] Referring to FIG. 9, the optical transmitter 111G may receive an
optical signal having a desired wavelength λ using ASE, and
modulate an intensity of the optical signal according to a transmission
data signal. The optical transmitter 111G includes a light source 901, a
wavelength demultiplexer 902, and a plurality of wavelength control units
914 and 924. The wavelength control units 914 and 924 may be called
modulators 914 and 924. The optical transmitter 111G may be connected to
the optical channel 121 through the wavelength multiplexer 112 using an
AWG.

[0109] ASE may be used as the light source 901. ASE may have a relatively
wide wavelength band, as shown in FIG. 4. Since ASE is used as the light
source 901, manufacturing costs of the optical transmitter 111G may be
reduced and a process of stabilizing the light source 901 may be not
required.

[0110] The wavelength demultiplexer 902 may use an AWG. The wavelength
demultiplexer 902 may receive an optical signal from the light source 901
and divide the optical signal according to the wavelengths λ1,
. . . , and λn. Optical signals passing through the wavelength
demultiplexer 902 may be distributed to the plurality of channels CH1, .
. . , and CHn. That is, the first channel CH1 selects an optical signal
of the first wavelength λ1 and combines the optical signal of
the first wavelength λ1 with the first transmission data
signal ES1, and the nth channel CHn selects an optical signal of the
nth wavelength λn and combines the optical signal of the
nth wavelength λn with the nth transmission data
signal ESn.

[0111] The wavelength control unit 914 of the first channel CH1 may
modulate an intensity of an optical signal transmitted through the
wavelength demultiplexer 902 according to the first transmission data
signal ES1. The wavelength control unit 914 may include a first waveguide
903, a first prototype filter 904, a second waveguide 906, a third
waveguide 907, and a modulator 911. An optical signal input to the
wavelength control unit 914 may be transmitted through the first
waveguide 903. An optical signal transmitted through the first waveguide
903 may be transmitted to the first prototype filter 904. The first
prototype filter 904 may have a resonance wavelength of, for example, the
first wavelength λ1. An optical component having the first
wavelength λ1 that matches a resonance curve of the first
prototype filter 904 may be output to the second waveguide 906.

[0112] An optical signal of the first wavelength λ1 output from
the first prototype filter 904 may be transmitted to the second waveguide
906 and the third waveguide 907. An optical signal transmitted to the
second waveguide 906 may be provided to the modulator 911 to be optically
modulated. The modulator 911 may modulate an intensity of the optical
signal according to the first transmission data signal ES1, which is an
electrical signal.

[0113] An optical signal transmitted to the third waveguide 907 may be
used to monitor a power state of the light source 901. In detail, when
the power of the optical signal transmitted to the fourth waveguide 907
is lower than an upper limit, the power of the light source 901 may be
increased, and when the power of the optical signal transmitted to the
third waveguide 907 is higher than the upper limit, the power of the
light source 901 may be reduced. The power of the light source 901 may be
monitored without interrupting its transmission.

[0114] The modulator 911 may include a second prototype filter 908, a
first electrode 909, and a second electrode 910. The second prototype
filter 908 may have a resonance wavelength of the first wavelength
λ1, like the first prototype filter 904. The first electrode
909 may be disposed on an outer circumferential surface of the second
prototype filter 908, and the second electrode 910 may be disposed on an
inner circumferential surface of the second prototype filter 908. The
first transmission data signal ES1 is a binary signal that may be applied
to the first electrode 909 and the second electrode 910.

[0115] The second prototype filter 908 may receive an optical signal
transmitted through the second waveguide 906 and outputs an optical
component having the first wavelength λ1 that matches a
resonance curve of the second prototype filter 908. An intensity of an
optical signal of the first wavelength λ1 output from the
second prototype filter 908 may be modulated by to a voltage difference
between the first and second electrodes 909 and 910 due to the first
transmission data signal ES1. When the first transmission data signal ES1
is logic low with no voltage difference between the first and second
electrodes 909 and 910, an intensity of an optical signal output from the
second prototype filter 908 is maximized. When the first transmission
data signal ES1 is logic high with a predetermined voltage difference
between the first and second electrodes 909 and 910, an intensity of an
optical signal output from the second prototype filter 908 is minimized.

[0116] An optical signal modulated according to the first transmission
data signal ES1 and output from the second prototype filter 908 may be
transmitted to the wavelength multiplexer 112 through a fourth waveguide
913. 100114] The first waveguide 903, the first prototype filter 904, the
second and third waveguides 906 and 907, and the modulator 911 may be
thermally coupled to one another to maintain the same temperature. A
refractive index of each of the first, second and third waveguides 903,
906 and 907 may vary due to an ambient temperature change. The first
wavelength λ1, the resonance wavelength of the first and
second prototype filters 904 and 908, may vary due to an ambient
temperature change.

[0117] To input a stably resonated optical signal of first wavelength
λ1 to the modulator 911, the thermally coupled first waveguide
903, the first prototype filter 904, the second and third waveguides 906
and 907, and the modulator 911 may be integrally formed as one wavelength
control unit 914 on a semiconductor substrate. A high thermal
conductivity material may be used in the wavelength control unit 914, or
a trench that intercepts external heat may be formed in a boundary
surface of the wavelength control unit 914 and be filled with a low
thermal conductivity material.

[0118] Like the first channel CH1, other channels of the optical
transmitter 111G may each include a wavelength control unit. For example,
in the nth channel CHn, the wavelength control unit 924 may modulate
an optical signal having the nth wavelength λn obtained
by the wavelength demultiplexer 902 according to the nth
transmission data signal ESn. Prototype filters in the wavelength control
unit 924 may have a resonance wavelength of, for example, the nth
wavelength λn.

[0119] The wavelength control unit 924 may receive an optical signal of
the nth wavelength λn from the wavelength demultiplexer
902 and modulate the optical signal according to the nth
transmission data signal ESn. An optical signal modulated according to
the nth transmission data signal ESn and output from the wavelength
control unit 924 may be transmitted to the wavelength multiplexer 112
through a waveguide 923.

[0120] The wavelength control unit 924 may be integrally formed on a
semiconductor substrate to maintain the same temperature for the
waveguides and prototype filters in the wavelength control unit 924. A
high thermal conductivity material may be used in the wavelength control
unit 924, or a trench that intercepts external heat may be formed in a
boundary surface of the wavelength control unit 924 and be filled with a
low thermal conductivity material.

[0121] While embodiments of the inventive concept has been particularly
shown and described with reference to exemplary embodiments thereof using
specific terms, the embodiments and terms used herein should not be
construed as limiting the scope of embodiments of the inventive concept
defined by the claims. Accordingly, it will be understood by those of
ordinary skill in the art that various changes in form and details may be
made therein without departing from the spirit and scope of the inventive
concept as defined by the following claims.